JP5473445B2 - Epitaxial wafer - Google Patents

Description

  The present invention relates to an epitaxial wafer, and more particularly to an epitaxial wafer on a Si substrate having a two-dimensional electron gas made of a nitride III-V compound semiconductor.

  Conventionally, a heterostructure using a nitride III-V compound semiconductor AlGaN / GaN has been grown on a sapphire or Si substrate because the GaN substrate is expensive.

  In the growth of nitride III-V compound semiconductors on Si substrates, various buffer layer structures are used to reduce differences in crystal structure with the substrate, lattice mismatch with the substrate, and differences in thermal expansion coefficient with the substrate. It is used.

  When a nitride III-V compound semiconductor is grown on a Si substrate, the most commonly used buffer layer is a buffer composed of a multilayer film described in JP-A-2005-85852 (Patent Document 1). Is a layer.

  On the AFM (Atomic Force Microscope) observation surface of the epitaxial wafer having a buffer layer structure composed of such a multilayer film, there are continuous defects as shown in FIG. This is because the lattice constants of each layer of GaN / AlN or AlGaN / AlN used for the buffer layer made of a multilayer film are greatly different, so that island growth is adopted during the growth process, and the island-island fusion part Such defects appear due to the occurrence of defects. The (004) plane X-ray half-width of GaN grown on the Si substrate via a multilayer buffer layer is about 800 arcsec due to this defect. In addition, in the epitaxial wafer when the buffer layer composed of the multilayer film is used, the GaN layer as the electron transit layer formed on the buffer layer increases the warpage of the downwardly convex wafer. It can only be about 1 μm, and when the GaN layer is formed, the wafer warps by several tens of μm in a convex shape downward.

  As one of the buffer layers other than the buffer layer composed of the multilayer film, a buffer layer having a composition gradient disclosed in JP-T-2004-524250 (Patent Document 2) is used.

  The characteristic of the buffer layer with the composition gradient is that continuous defects such as a buffer layer composed of a multilayer film are not observed, and therefore, the (004) plane X-ray half width of the GaN layer grown on the buffer layer is about 600 arcsec. It becomes smaller than the case of the buffer layer made of a multilayer film. However, the epitaxial wafer in the case of the buffer layer having such a gradient composition also has a GaN layer thickness of 1 μm because the GaN layer as the electron transit layer formed on the buffer layer increases the warp of the downwardly convex wafer. It can grow only to a certain extent, and when the GaN layer is formed, the wafer warps to several tens of μm in a downwardly convex shape.

  As described above, in the current epitaxial wafer, when a nitride III-V compound semiconductor is grown on the buffer layer, the (004) plane X-ray half width is improved and the warpage of the wafer itself is minimized. Is difficult. In addition, it is difficult to increase the thickness of the GaN single layer as the electron transit layer formed on the buffer layer.

JP-A-2005-85852 Special table 2004-524250 gazette

  Accordingly, an object of the present invention is to make the wafer warp extremely small while reducing crystal defects, compared to the case of using a buffer layer having a conventional structure, and an epitaxial wafer capable of increasing the thickness of the GaN layer formed on the buffer layer. Is to provide.

In order to solve the above problems, the epitaxial wafer of the present invention is
A Si substrate;
A buffer layer formed on the Si substrate;
A composition gradient layer made of AlGaN formed on the buffer layer;
An electron transit layer made of GaN formed on the composition gradient layer;
An electron supply layer formed on the electron transit layer,
The buffer layer has _{a first} composition formula Al _{x → a} Ga _{1−x → 1-a} N in which the Al composition ratio continuously changes from x to a (> x) in the thickness direction from the Si substrate side. And at least two layers of a composition formula Al _{a → x} Ga _{1-a → 1-x} N in which the Al composition ratio continuously changes from a to x in the thickness direction from the Si substrate side. , Having a multilayer structure part that is repeatedly laminated in the order of the first layer and the second layer,
In the composition gradient layer, the Al composition ratio continuously decreases in the thickness direction from the uppermost layer of the buffer layer toward the electron transit layer side ,
The thickness of the electron transit layer is set such that the warpage of the wafer at room temperature is substantially zero .

  When a buffer layer composed of a multilayer film is used, a continuous defect as shown in FIG. 4 occurs. However, the buffer layer has a multilayer structure in which AlGaN layers whose composition is continuously changed are repeatedly laminated as in the present invention. By inserting, island growth is suppressed, and as a result, continuous defects are eliminated and crystallinity is improved.

  In both the case of the above-mentioned Patent Document 1 (Japanese Patent Laid-Open No. 2005-85852) and the case of Patent Document 2 (Japanese Patent Publication No. 2004-524250), the warpage of the wafer has already protruded downward at the time of buffer layer growth. Yes. On the other hand, according to the epitaxial wafer having the above-described structure, when the multilayer structure portion of the buffer layer is formed, the warpage of the wafer becomes convex upward, and the electron transit layer on the multilayer structure portion causes the warpage of the wafer. Since it cancels out by projecting downward, the thickness of the electron transit layer can also be made significantly thicker than in the case of both Patent Documents 1 and 2. Further, since the size of the convex warp on the wafer increases with the repetition period, the thicker the multilayer structure portion of the buffer layer, the thicker the electron transit layer can be grown. This is an unexpected effect.

  Furthermore, the gradient composition of Patent Document 2 is only that the Al composition decreases from the substrate side to the surface side, but according to the epitaxial wafer of the present invention, it differs in that the Al composition is continuously and periodically inclined. ing.

  Further, when an electron transit layer having a different composition is directly grown on the buffer layer, the electron transit layer begins to gradually relax the lattice from the interface with the buffer layer. This relaxation layer is not desirable because it becomes a new source of piezo charges and consequently acts as a new leak path. In the epitaxial wafer of the present invention, the occurrence of a leak path can be suppressed by the presence of the composition gradient layer made of AlGaN whose composition is continuously changed between the buffer layer and the electron transit layer.

  Therefore, the warpage of the wafer can be made extremely small while reducing crystal defects as compared with the case where the buffer layer having the conventional structure is used, and the GaN layer formed on the buffer layer can also be made thicker.

  The epitaxial wafer of the present invention is suitable for a heterojunction field effect transistor made of a nitride III-V compound semiconductor.

Further, the warpage of the wafer is caused by stress caused by the thermal expansion coefficient between the substrate and the film, and the physical constants of the substrate (thickness t, diameter D, Young's modulus E, Poisson's ratio ν), warpage h, When the film thickness is L, the film stress σ is
σ = 8E · t ² · h / (6 (1-ν) D ² L)
It is represented by Since the stress σ of the film is constant as long as the thickness L of the film does not change, the warpage h inevitably increases as the thickness t of the substrate decreases by polishing. That is, when the film stress σ is large, the warpage of the wafer is further increased by polishing the substrate. In contrast, when the warp is theoretically zero (the film stress is zero), the warp remains zero even if the thickness t of the substrate is reduced by polishing. Therefore, the epitaxial wafer of the present invention can realize an epitaxial wafer in which the warpage of the wafer does not increase even when the substrate is polished.

In the epitaxial wafer of one embodiment,
The multilayer structure of the buffer layer is
Two layers of the first layer and the second layer are alternately and repeatedly stacked in the order of the first layer and the second layer.

  According to the above-described embodiment, it is possible to improve the crystallinity and reduce the warpage of the wafer by using the multilayer structure portion having only two AlGaN layers having two graded compositions, and to reduce the thickness of the entire buffer layer.

In the epitaxial wafer of one embodiment,
The multilayer structure of the buffer layer is
A third layer of composition formula Al _a Ga _1-a N (0 ≦ a ≦ 1) and a fourth layer of composition formula Al _x Ga _1-x N (0 ≦ x ≦ 1);
The four layers of the first layer, the third layer, the second layer, and the fourth layer are the first layer, the third layer, the second layer, and the fourth layer. Laminated repeatedly in the order of the layers.

  According to the embodiment, the multilayer structure portion of the buffer layer can be thickened to increase the wafer strength.

In the epitaxial wafer of one embodiment,
The buffer layer
An AlN layer formed on the Si substrate;
An AlGaN layer formed on the AlN layer and having an Al composition ratio continuously changing from 1 to (x + a) / 2;
The multilayer structure is formed on the AlGaN layer.

  Two layers of AlGaN / AlN are usually grown under the buffer layer made of a multilayer film having a conventional periodic structure. The difference in the lattice constant of this part is also a cause of island growth and new dislocations. Is generated. Therefore, according to the above embodiment, in order to reduce this dislocation, after the growth of the AlN layer, the AlGaN whose composition is continuously changed from 100% Al composition to the average composition (x + a) / 2 of the multilayer structure of the buffer layer. By growing the layer, generation of defects between the AlGaN layer and the multilayer structure of the buffer layer can be further suppressed.

In the epitaxial wafer of one embodiment,
In the composition gradient layer, the Al composition ratio continuously changes in the thickness direction from the buffer layer side until the Al composition ratio becomes zero from the Al composition ratio of the uppermost layer of the buffer layer.

  According to the embodiment, the composition gradient layer formed between the buffer layer and the electron transit layer is formed in the thickness direction from the buffer layer side until the Al composition ratio becomes zero from the Al composition ratio of the uppermost layer of the buffer layer. By continuously changing the Al composition ratio, the crystallinity of the electron transit layer made of GaN formed on the composition gradient layer can be further improved.

  As is clear from the above, according to the epitaxial wafer of the present invention, the warpage of the wafer can be made extremely small while reducing crystal defects, compared to the case of using the buffer layer having the conventional structure, and GaN formed on the buffer layer. An epitaxial wafer in which the layer can be thickened can be realized.

FIG. 1 shows a method of manufacturing an epitaxial wafer according to the first embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the number of periods and warpage of the multilayer structure portion of the buffer layer. FIG. 3 shows an epitaxial wafer manufacturing method according to the second embodiment of the present invention. FIG. 4 is an AFM image of GaN grown on a buffer layer composed of a multilayer film having a conventional structure.

  Hereinafter, an epitaxial wafer of the present invention will be described in detail with reference to the illustrated embodiments.

[First Embodiment]
FIG. 1 shows an epitaxial wafer manufacturing method according to the first embodiment of the present invention.

  As shown in FIG. 1, a 3 inch Si substrate 1 is used as the substrate. Prior to the growth, the surface oxide film of the Si substrate 1 is removed with a hydrofluoric acid-based etchant, and then the Si substrate 1 is set in a MOCVD (Metal Organic Chemical Vapor Deposition) apparatus.

  The substrate temperature is set to 1100 ° C., and the substrate surface is cleaned at a chamber pressure of 13.3 kPa.

The substrate temperature and the chamber pressure are kept constant, and the surface of the Si substrate 1 is nitrided by flowing ammonia NH ₃ (12.5 slm). Subsequently, the AlN layer 2 (trimethylaluminum TMA flow rate = 125 μmol / min, NH ₃ flow rate = 12 .5 slm)
For 100 nm,
Al _0.2 Ga _0.8 N layer 3 (TMA flow rate = 50 μmol / min, TMG flow rate = 100 μmol, NH ₃ flow rate = 12.5 slm)
For 40 nm.

after that,
Al _{0.2 → 1} GaN as the first layer (TMA flow rate = 50 → 125 μmol / min, TMG flow rate = 0 → 100 μmol, NH ₃ flow rate = 12.5 slm) / AlN as the third layer (TMA flow rate = 125 μmol / min, NH ₃ flow rate = 12.5 slm) / Al _{1 → 0.2} GaN as the second layer (TMA flow rate = 125 → 50 μmol / min, TMG flow rate = 100 → 0 μmol, NH ₃ flow rate = 12.5 slm ) / Al _0.2 GaN as the fourth layer (TMA flow rate = 50 μmol / min, TMG flow rate = 100 μmol, NH ₃ flow rate = 12.5 slm) Multilayer structure 4 (5/5/5/25 nm, 50 cycles) ,
Al _{0.2 → 0} GaN (TMA flow rate = 50 _{→ 0} μmol / min, TMG flow rate = 100 → 50 μmol, NH ₃ flow rate = 12.5 slm) composition gradient layer 5 (50 nm),
GaN (TMG flow rate = 50 μmol, NH ₃ flow rate = 12.5 slm) electron transit layer 6 (1.5 μm),
Al _0.2 Ga _0.8 N barrier layer 7 (20 nm) as an electron supply layer
Grow sequentially.

In the first embodiment, the buffer layer has a composition formula Al _{x → a} Ga in which the Al composition ratio continuously changes from x = 0.2 to a = 1 (> x) in the thickness direction from the Si substrate 1 side. _{1-x → 1-a} N first layer and composition formula Al _{a → x} Ga _{1−a → 1- in} which the Al composition ratio continuously changes from a to x in the thickness direction from the Si substrate 1 side. _a second layer of _xN, a third layer of composition formula Al _a Ga _1-a N (0 ≦ a ≦ 1), and a third layer of composition formula Al _x Ga _1-x N (0 ≦ x ≦ 1). Four of the four layers have the multilayer structure portion 4 that is repeatedly laminated in the order of the first layer, the third layer, the second layer, and the fourth layer. The AlN layer 2, the Al _0.2 Ga _0.8 N layer 3 and the multilayer structure 4 constitute a buffer layer.

  The Al composition of the AlGaN layer 3 is not particularly limited. Further, the Al composition and thickness of the AlGaN barrier layer 7 are not particularly limited, and can be changed according to the required sheet carrier concentration.

  FIG. 2 shows the relationship between the number of periods and warpage of the multilayer structure portion of the buffer layer. In FIG. 2, the horizontal axis represents the number of periods of the multilayer structure 4, and the vertical axis represents warpage (μm).

As shown in FIG. 2, the warpage at room temperature increases as the number of periods of AlN / Al _{1 → 0.2} GaN / Al _0.2 GaN / Al _{0.2 → 1} GaN multilayer structure 4 increases. This tendency is different from the case of AlN / Al _0.2 GaN (in the case of no composition gradient intermediate layer) (in the direction of warping).

  By optimizing the thickness of the GaN electron transit layer 6 grown on the multilayer structure 4, the wafer warpage at room temperature can be made substantially zero. For example, since the wafer warpage is approximately 40 μm downward with respect to GaN having a thickness of 1 μm, if the wafer warp after the growth of the buffer layer is upwardly convex and 60 μm, the GaN electron transit layer 6 having a thickness of 1.5 μm. When the wafer is grown, the warpage of the wafer is canceled and becomes almost zero.

  By growing the GaN electron transit layer 6 having a thickness of about 1.5 μm with respect to the cycle number 50 of the multilayer structure portion of the buffer layer, the warpage of the wafer at room temperature can be suppressed within ± 5 μm. In addition, the (004) plane X-ray half-width was improved to about 450 arcsec over the conventional structure.

  For a cycle number of 100, the GaN electron transit layer 6 can be grown to approximately 3 μm, and its (004) plane X-ray half-width is about 300 arcsec.

  Thus, according to the epitaxial wafer of the first embodiment, the warpage of the wafer can be made extremely small while reducing crystal defects, compared with the case where the buffer layer having the conventional structure is used, and the GaN formed on the buffer layer. The electron transit layer can also be thickened.

  Further, according to the epitaxial wafer, the multilayer structure portion of the buffer layer can be thickened to increase the wafer strength.

  In the first embodiment, after the growth of the AlN layer, by growing an AlGaN layer whose composition is continuously changed from 100% Al composition to the average composition (x + a) / 2 of the multilayer structure of the buffer layer, It is possible to further suppress the occurrence of defects between the multilayer structure portion of the buffer layer.

In particular,
(x + a) / 2 = (0.2 + 1) /2=0.6
By adding an AlN → Al _0.6 Ga _0.4 N composition gradient layer on the AlN layer 2 immediately above the Si substrate 1, the occurrence of defects between the AlN layer and the multilayer structure is further reduced. The (004) plane X-ray half width can be further reduced by about 50 to 100 arcsec.

  Further, the composition gradient layer 5 formed between the buffer layer and the electron transit layer 6 has an Al composition ratio in the thickness direction from the buffer layer side until the Al composition ratio becomes zero from the Al composition ratio of the uppermost layer of the buffer layer. By continuously changing, the crystallinity of the electron transit layer 6 made of GaN formed on the composition gradient layer 5 can be further improved.

[Second Embodiment]
FIG. 3 shows an epitaxial wafer manufacturing method according to the second embodiment of the present invention.

  In the second embodiment of the present invention, as in the first embodiment, a 3-inch Si substrate 1 is used as the substrate. Prior to the growth, the surface oxide film of the Si substrate 1 is removed with a hydrofluoric acid-based etchant, and then the Si substrate 1 is set in the MOCVD apparatus.

  The substrate temperature is set to 1100 ° C., and the substrate surface is cleaned at a chamber pressure of 13.3 kPa.

Nitridation of the Si substrate surface is performed by flowing ammonia NH ₃ (12.5 slm) while keeping the substrate temperature and chamber pressure constant.
AlN layer 12 (TMA flow rate = 125 μmol / min, NH ₃ flow rate = 12.5 slm)
For 150 nm,
Al _0.3 Ga _0.7 13 (TMA flow rate = 70 μmol / min, TMG flow rate = 90 μmol, NH ₃ flow rate = 12.5 slm)
For 40 nm.

after that,
Al _{0.3 → 1} GaN as the first layer (TMA flow rate = 50 → 125 μmol / min, TMG flow rate = 100 → 0 μmol, NH ₃ flow rate = 12.5 slm) / Al _{1 →} 2 as the second layer _{. 3} GaN (TMA flow rate = 125 → 50 μmol / min, TMG flow rate = 0 → 100 μmol, NH ₃ flow rate = 12.5 slm) multilayer structure 14 (5/5 nm, 50 cycles),
Al _{0.3 → 0} GaN (TMA flow rate = 50 _{→ 0} μmol / min, TMG flow rate = 100 → 50 μmol, NH ₃ flow rate = 12.5 slm) composition gradient layer 15 (50 nm),
GaN (TMG flow rate = 50 μmol, NH ₃ flow rate = 12.5 slm) electron transit layer 16 (1.5 μm),
Al _0.25 Ga _0.75 N barrier layer 17 (20 nm) as an electron supply layer
Grow sequentially.

In the second embodiment, the buffer layer has a composition formula Al _{x → a} Ga in which the Al composition ratio continuously changes from x = 0.3 to a = 1 (> x) in the thickness direction from the Si substrate 1 side. _{1-x → 1-a} N first layer and composition formula Al _{a → x} Ga _{1−a → 1- in} which the Al composition ratio continuously changes from a to x in the thickness direction from the Si substrate 1 side. Two layers of the _xN second layer have the multilayer structure portion 14 that is repeatedly stacked in the order of the first layer and the second layer. The AlN layer 12, the Al _0.3 Ga _0.7 N layer 13 and the multilayer structure portion 14 constitute a buffer layer.

  The Al composition of the AlGaN layer 13 is not particularly limited. Further, the Al composition and thickness of the AlGaN barrier layer 17 are not particularly limited, and can be changed according to the required sheet carrier concentration.

  By optimizing the thickness of the GaN electron transit layer 16 grown on this buffer layer, the warpage of the wafer at room temperature can be made substantially zero. For example, since the wafer warpage is approximately 40 μm downward with respect to GaN having a thickness of 1 μm, if the wafer warp after the growth of the buffer layer is convex upward and 60 μm, the GaN electron transit layer 16 of 1.5 μm is formed. When the wafer is grown, the warpage of the wafer is canceled and becomes almost zero.

  By growing the GaN electron transit layer 16 of about 1.5 μm with respect to the period number 50 of the multilayer structure portion of the buffer layer, it becomes possible to suppress the warpage of the wafer at room temperature within ± 5 μm. Also, the (004) plane X-ray half width was about 450 arcsec, which was improved from the conventional structure.

  For a cycle number of 100, the GaN electron transit layer 16 of about 3 μm can be grown, and its (004) plane X-ray half width was about 300 arcsec.

  In the first embodiment, a GaN electron transit layer having a thickness of 40 nm × 50 = 2 μm and a thickness of 1.5 μm is grown. In the second embodiment, the same effect is obtained with a thickness of 10 nm × 50 = 0.5 μm. Was realized.

  Thus, according to the epitaxial wafer of the second embodiment, the warpage of the wafer can be made extremely small while reducing crystal defects, compared with the case where the buffer layer having the conventional structure is used, and the GaN formed on the buffer layer. The electron transit layer can also be thickened.

  In the second embodiment, after the growth of the AlN layer, by growing an AlGaN layer having a composition changed continuously from an Al composition of 100% to an average composition (x + a) / 2 of the multilayer structure of the buffer layer, It is possible to further suppress the occurrence of defects between the multilayer structure portion of the buffer layer.

In particular,
(x + a) / 2 = (0.3 + 1) /2=0.65
By adding an AlN → Al _0.65 Ga _0.35 N composition gradient layer on the AlN layer 12 immediately above the Si substrate 11, the occurrence of defects between the AlN layer and the multilayer structure is further reduced. The (004) plane X-ray half width can be further reduced by about 50 to 100 arcsec.

  In addition, the composition gradient layer 15 formed between the buffer layer and the electron transit layer 16 has an Al composition ratio in the thickness direction from the buffer layer side until the Al composition ratio becomes zero from the Al composition ratio of the uppermost layer of the buffer layer. By continuously changing, the crystallinity of the electron transit layer 16 made of GaN formed on the composition gradient layer 15 can be further improved.

  As described above, according to the present invention, the (004) plane X-ray half width is smaller than that in the case of using a buffer layer having a conventional structure, the warpage of the wafer is extremely small, and a thicker GaN layer can be grown. In addition, an epitaxial wafer can be realized in which the warpage of the wafer does not increase even when the substrate is polished.

  The epitaxial wafer of the present invention is suitable for a heterojunction field effect transistor made of a nitride III-V compound semiconductor.

  Although specific embodiments of the present invention have been described, the present invention is not limited to the first and second embodiments described above, and various modifications can be made within the scope of the present invention.

DESCRIPTION OF SYMBOLS 1,11 ... Si substrate 2,12 ... AlN layer 3 ... Al _0.2 Ga _0.8 N layer 4 ... AlN / Al _{1 → 0.2} GaN / Al _0.2 GaN / Al _{0.2 → 1} GaN multilayer Structure 5 ... Al _{0.2 → 0} GaN inclined layer 6,16 ... GaN electron transit layer 7 ... Al _0.2 Ga _0.8 N barrier layer 13 ... Al _0.3 Ga _0.7 N layer 14 ... Al _{1 → 0.3} GaN / Al _{0.3 → 1} GaN multilayer structure 15... Al _{0.3 → 0} GaN inclined layer 17... Al _0.25 Ga _0.75 N barrier layer

Claims (5)

A Si substrate;
A buffer layer formed on the Si substrate;
A composition gradient layer made of AlGaN formed on the buffer layer;
An electron transit layer made of GaN formed on the composition gradient layer;
An electron supply layer formed on the electron transit layer,
The buffer layer has _{a first} composition formula Al _{x → a} Ga _{1−x → 1-a} N in which the Al composition ratio continuously changes from x to a (> x) in the thickness direction from the Si substrate side. And at least two layers of a composition formula Al _{a → x} Ga _{1-a → 1-x} N in which the Al composition ratio continuously changes from a to x in the thickness direction from the Si substrate side. , Having a multilayer structure part that is repeatedly laminated in the order of the first layer and the second layer,
In the composition gradient layer, the Al composition ratio continuously decreases in the thickness direction from the uppermost layer of the buffer layer toward the electron transit layer side ,
An epitaxial wafer characterized in that the thickness of the electron transit layer is set so that the warpage of the wafer at room temperature is substantially zero . The epitaxial wafer according to claim 1,
The multilayer structure of the buffer layer is
An epitaxial wafer, wherein two layers of the first layer and the second layer are alternately and repeatedly laminated in the order of the first layer and the second layer. The epitaxial wafer according to claim 1,
The multilayer structure of the buffer layer is
A third layer of composition formula Al _a Ga _1-a N (0 ≦ a ≦ 1) and a fourth layer of composition formula Al _x Ga _1-x N (0 ≦ x ≦ 1);
The four layers of the first layer, the third layer, the second layer, and the fourth layer are the first layer, the third layer, the second layer, and the fourth layer. An epitaxial wafer characterized by being laminated repeatedly in the order of layers. In the epitaxial wafer as described in any one of Claim 1 to 3,
The buffer layer
An AlN layer formed on the Si substrate;
An AlGaN layer formed on the AlN layer and having an Al composition ratio continuously changing from 1 to (x + a) / 2;
An epitaxial wafer comprising the multi-layer structure formed on the AlGaN layer. In the epitaxial wafer according to any one of claims 1 to 4,
The composition gradient layer is characterized in that the Al composition ratio continuously changes in the thickness direction from the buffer layer side until the Al composition ratio becomes zero from the Al composition ratio of the uppermost layer of the buffer layer. Epitaxial wafer.